Lactide is polymerizable to high molecular weight polylactic acids which are of great interest for their hydrolytic and biodegradable properties. For example, they have long been of interest for such biomedical uses as sutures and staples. More recently, they have become of interest for the manufacture of articles of commerce for non-biomedical uses that would be degradable in the environment, in particular hydrolytically, to environmentally acceptable products. For most, if not all such uses, it is preferred the degradable polymer be made from L-lactide However, lactide made by existing technology is too costly for such non-medical uses because of low yields and by-product information.
Lactide is most conveniently prepared by polymerizing lactic acid to a relatively low molecular weight (oligomeric) polylactic acid, then heating the oligomer, generally in the presence of a catalyst, as is well known in the art, to depolymerize it to lactide which is recovered as a component of a vapor product stream. A discussion of conventional methods for producing lactide can be found in the following documents: Gruter et al., U.S. Pat. No. 1,095,205 (1914); Lowe, U.S. Pat. No. 2,668,162 (1954); Bhatia, U.S. Pat. No. 4,835,293 (1989); Bellis U.S. Pat. No. 4,727,163 (1988); Muller, Ger. Patent Application Publication Numbers 3632103 and 3708915 (1988).
Such processes suffer in that they require rather long reaction times at high temperatures for the depolymerization reaction, and generate several waste streams. The lactide product stream contains by-product lactic acid and low molecular weight polylactic acid which constitute a yield loss. Since the polymer-to-lactide yields are generally low, it is desirable to be able to recover the lactic and polylactic acids from the product stream and recycle them for the production of additional lactide.
Further, the rather long residence times at the high temperatures employed often results in side reactions, leading, for example, to unwanted isomers, including meso lactide, charring of the polymer and consequently difficult to handle reactor heels.
The reactor heels also constitute a yield loss inasmuch as continued heating under depolymerization conditions generally fails to generate sufficient additional lactide to make such operation economically feasible. Further, it results in increased formation of the unwanted meso-isomer and other by-products which further complicate the downstream recovery operation.
Bhatia U.S. Pat. No. 4,835,293 discloses a gas-assisted depolymerization process for the production of dimeric cyclic esters such as lactide wherein a stream of an inert gas is employed to strip the cyclic ester from the reaction zone along with any volatile hydroxycarboxylic acid also formed therein. The resulting gaseous product stream is scrubbed with a polar organic solvent to recover the cyclic ester. The solvents include alcohols, ethers, esters and ketones, with use of isopropyl alcohol exemplifying the recovery of glycolide from its impurities. Isopropyl alcohol as scrubbing solvent solubilizes the hydroxycarboxylic acids and any water of reaction, thereby enabling the recovery of glycolide directly from the scrubbing medium as a substantially insoluble filterable crystalline solid.
Use of an alcohol, however, as the scrubbing solvent for the recovery of L-lactide from a vapor product stream is not entirely satisfactory. It reacts in the alcoholic solution to form alkyl lactate, which not only constitutes a yield loss but further increases the solubility of lactide in the scrubbing solution, further aggravating the yield loss problem. Also, since the starting L-lactic acid used to make lactide always contains some D-lactic acid, the reaction product always contains some meso-isomer. Meso-lactide is more soluble in alcohol than L-lactide and tends to increase the solubility of the L-isomer. Thus, when the lactic acid values are recovered from the alcoholic filtrate they are accompanied by the meso lactide, which continues to build up in the system and eventually results in greater solubility losses of L-lactide and decreased efficiency of the process.
On the other hand, use of a non-hydroxylic scrubbing solvent such as acetone, for example, which is non-reactive towards lactide and in which lactide is highly soluble, likewise presents difficulties inasmuch as such polar solvent solubilizes the by-product hydroxycarboxylic acids as well, so that further processing would be required to separate the lactide from the acids.
Water as a scrubbing solvent is also unsatisfactory in that heat transfer to it is much faster then mass transfer; consequently, lactide precipitates as fog of particles, difficult to capture in the absence of specialized and costly equipment.
Thus, a need exists for a means that provides for the substantially complete recovery of lactide from a vapor stream that also contains lactic acid and open-chain dimer, and at the same time provides for the substantially complete recovery of the acid values for recycle, in a way that avoids the build-up of the meso-isomer in the system. Also, there is a need for a process for the recovery of potential lactic acid values in polylactic acid residues that have little or no value as lactide sources in the depolymerization reaction.
It is an object of this invention to provide processes that meet the above needs. It is another object to provide an integrated high yield lactide process wherein the lactic acid values removed from the crude lactide vapor stream and recovered from the residual polylactic acid are recycled for the production of additional quantities of lactide.